Rotating coefficient filter

ABSTRACT

A circuit that provides a rotating coefficient FIR filter with all necessary coefficient sets present at the same time, without the need for delay elements or devices providing for adjustable impedances is described. An input signal is sampled in round robin fashion by a plurality of sample and hold devices. The outputs of the sample and hold devices are connected to sets of impedance devices. Each set of impedance devices implements the coefficients of the desired frequency response of the filter. The impedance devices in each set are connected to the sample and hold devices in a different order from each other set, so that each set of impedance devices will produce the desired frequency response when a different one of the sampling circuits contains a new sample of the input signal. Switches connect the sets of impedance devices to an output, only one switch being closed at a time to provide the output signal.

This application claims priority from Provisional Application No.61/613,911, filed Mar. 21, 2012, which is incorporated herein byreference in its entirety.

FIELD OF THE INVENTION

The present invention relates generally to electronic filters, and moreparticularly to filters that provide the functionality of finite impulseresponse (FIR) filters.

BACKGROUND OF THE INVENTION

A finite impulse response (FIR) filter is a type of electronic filterwith a broad range of applications. FIR filters are widely used in bothdigital signal processing and digital video processing, and theirconstruction is well known in the prior art.

One type of FIR filter is a transversal filter, or tapped delay linefilter, in which successively delayed versions of an input signal aremultiplied by certain coefficient values and then summed. The output ofsuch a filter is thus a weighted combination of voltages taken fromuniformly spaced taps.

FIG. 1 shows one example of such a FIR filter, taken from U.S. Pat. No.7,188,135. An input signal x(t) is fed to a line of delay elements 104,106, 108, etc., each of which introduces a delay of a predetermined timesuch that the output of delay element 104 is the input signal x(t-1),the output of delay element 106 is x(t-2), etc. The original signal x(t)is multiplied by a coefficient C₁, and each subsequent delayed versionof the input signal x(t) is multiplied by a coefficient C₂, C₃, etc.Element 124 is a summation means that adds all of the delayed andmultiplied signals to create output y(t) Such a filter is considered tobe of the Mth order, where M−1 is the number of delay elements.

By properly selecting the coefficient values, a FIR filter is designedto provide an output with a desired frequency response. The coefficientvalues are typically calculated by a software program which takes thedesired frequency response as an input.

In practice, FIR filters are often made using resistors to provide thecoefficients. Such a filter is shown in FIG. 2. Here filter 200 againcontains a plurality (7 are shown) of unit delay elements U1 to U7, eachof which introduces a predetermined delay of time. As above, the filteris considered to be an 8^(th) order filter, since 8 minus 1 is thenumber of delay elements, although FIR filters may have many moreelements when more coefficients are needed to provide the desiredfrequency response; over 100 coefficients, and thus elements, is notuncommon.

The output of each of the delay elements U1 to U7 is connected to anelement having an impedance value, typically through some bufferingmeans, such as buffers Z1 to Z7; here, the elements having impedancevalues are shown as resistors R1 to R7. One of skill in the art willrecognize that while this example and the following discussion useresistors to indicate the impedance values for purposes of illustration,other circuit elements also have impedance values, for example,capacitors, inductors, depletion mode MOSFETs, and other devices, andany device having an impedance that does not otherwise interfere withoperation of the filter may be used to provide the desired impedancevalues as described herein.

The resistors R1 to R7 all share a common output point. As an inputsignal progresses through the delay elements, each resistor causes thesignal on the respective delay element to which it is attached tocontribute to the output signal in inverse proportion to the resistorvalue. Thus, if the resistor is small, the signal on the attached delayelement will have a large contribution to the output voltage, while ifthe resistor is large the contribution to the output will be smaller. Itis thus known in the art that by selecting impedance values that are theinverse of the desired coefficients, a circuit as shown in FIG. 2 usingresistors or other elements having impedance will effectively providethe multiplication by coefficients shown in FIG. 1.

However, delaying a signal is not an easy operation if the signal is ananalog quantity; it typically necessitates not just simple sample andhold devices, or more commonly charge coupled devices (CCDs), but morecomplex delay elements operating in a chain such that the samples arepassed from one delay element to the next. Accordingly, it would beadvantageous to be able to build a FIR filter without delay elements.

SUMMARY OF THE INVENTION

A circuit is disclosed, that provides a rotating coefficient FIR filterwith all necessary coefficient sets present at the same time, withoutthe need for delay elements or devices providing for adjustableimpedances.

A first embodiment discloses a circuit comprising: an input configuredto receive an input signal; a plurality of sampling circuits arranged inparallel for sampling the input signal in response to a timing signal,the sampling circuits configured such that successive sampling circuitscreate samples of the input signal in time-delayed succession atpre-determined intervals; a plurality of sets of elements havingimpedances, each set containing the same number of elements as the ontuber of sampling circuits with the impedances chosen so that the sum ofoutputs from the elements in each set produces a desired frequencyresponse to the input signal that is the same for each set, with eachelement in a set connected to a different one of the sampling circuitsin a different order from the connection of the elements in each otherset, such that each set of elements will produce the desired frequencyresponse when a different one of the sampling circuits contains as newsample of the input signal; and a plurality of switches, each switchconnecting one of the plurality of sets of elements to the output.

In another embodiment a method is disclosed of designing a finiteimpulse response filter having a plurality of sampling circuits arrangedin parallel for sampling the input signal in response to a timingsignal, the sampling circuits configured such that successive samplingcircuits create samples of the input signal in time-delayed successionat pre-determined intervals, comprising: selecting a desired frequencyresponse for the filter; selecting a plurality of sets of elementshaving impedances, each set containing the same number of elements asthe number of sampling circuits with the impedances chosen so that thesum of outputs from the elements in each set produces a desiredfrequency response to the input signal that is the same for all of thesets, with each element in a set to be connected to a different one ofthe sampling circuits in a different order from the connection of theelements in each other set, such that each set of elements will producethe desired frequency response when a different one of the samplingcircuits contains a new sample of the input signal; and for each set ofelements, providing a switch connected an output and to all of theelements in the set that, the switch being separate from the switchesconnected to the other sets of elements.

Another embodiment discloses a computer readable storage medium havingembodied thereon instructions for causing a computing device to executea method for designing a finite impulse response filter having aplurality of sampling circuits arranged in parallel for sampling theinput signal in response to a timing signal, the sampling circuitsconfigured such that successive sampling circuits create samples of theinput signal in time-delayed succession at pre-determined intervals, themethod comprising: selecting a desired frequency response for thefilter; selecting a plurality of sets of elements having impedances,each set containing the same number of elements as the number ofsampling circuits with the impedances chosen so that the sum of outputsfrom the elements in each set produces a desired frequency response tothe input signal that is the same for all of the sets, with each elementin a set to be connected to a different one of the sampling circuits ina different order from the connection of the elements in each other set,such that each set of elements will produce the desired frequencyresponse when a different one of the sampling circuits contains a newsample of the input signal; and for each set of elements, providing aswitch connected an output and to all of the elements in the set that,the switch being separate from the switches connected to the other setsof elements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of one embodiment of a prior art FIR filter.

FIG. 2 is a schematic diagram of one embodiment of a prior art FIRfilter.

FIG. 3 is a block diagram of one embodiment of a prior art round robinsampling circuit that can function as a rotating coefficient FIR filter.

FIG. 4 is a schematic diagram of one embodiment of a prior art rotatingcoefficient FIR filter.

FIG. 5 is a schematic diagram of one embodiment of a circuit thatprovides a rotating coefficient FIR filter without adjustable elementsand with a reduced number of switches.

FIG. 6 is a schematic diagram of another embodiment of a circuit thatprovides a rotating coefficient FIR filter without adjustable elementsand with a reduced number of switches in a case where the output isunder-sampled compared to the input.

DETAILED DESCRIPTION OF THE INVENTION

The present application, describes the design and implementation Of afilter that has the output of a finite impulse response (FIR) filterwithout the need for delay elements or a large number of switches.

One known way of avoiding passing an analog signal down a delay line isby the use of a so-called “rotating coefficient” FIR filter. In a FIRfilter of this type, the samples of the input signal are taken by aplurality of sample and hold circuits in a “round robin” fashion ratherthan being passed from one delay element to another. Once a sample isloaded into a given sample and hold circuit, it need not be passed tosuccessive sample and hold circuits since the coefficients are made torotate to the correct values as required.

U.S. Pat. No. 7,028,070 (“the '070 patent”), entitled “High SpeedFilter” (as well as the continuation of the '070 patent, U.S. Pat. No.8,001,172) describes such a circuit that operates by using a series ofsampling elements activated in a “round-robin” fashion and thus providesan alternative way of obtaining a discrete approximation to a Fourierintegral of a signal using a substantially analog signal processingarray. The '070 patent is incorporated herein by reference as though setforth in full.

FIG. 3 is a block diagram of one embodiment of such a prior art roundrobin circuit. Circuit 300 contains a plurality N of sample and holdamplifiers (SHAs) 32 and an equal. number N of multipliers 36. Thenumber N is considered to be the “length” of the filter. An equal numberN of identical optional filter elements 34 is shown; one of skill in theart will appreciate that such filter elements may be used in certainapplications, such as that shown in co-pending U.S. patent applicationSer. No. 13/668,253, commonly owned by the assignee of the presentapplication.

An input signal S_(IN) is applied to the SHAs 32 in parallel, ratherthan being passed from one SHA to the next, but each SHA 32 is activatedin sequence so as to operate in a round-robin order. Specifically, afirst SHA 32 (labeled SHA₀) is first activated to sample the signalS_(IN), then a second SHA 32 (SHA₁) is activated to sample S_(IN), thenSHA₂ is activated, etc, up to SHAN_(N). After all of the SHAs have beenactivated in sequence, SHA₀ is activated again, then SHA₁ is activatedagain, etc.

The output of each of these samples taken by the round-robin action ofthe SHAs 32 is fed to one of the filters 34 if the filters are used, andthen to a corresponding multiplier 36 as shown. In addition to thefiltered output, each of the N multipliers receives a second input valueC₀ to C_(N) representing a coefficient of the Fourier seriesapproximation of a desired signal to be mixed with the input signal.

It will be appreciated that each SHA 32 may now operate at a lowerfrequency than would be required for a single analog-to-digitalconverter (ADC) in a traditional FIR filter; specifically, the desiredsampling interval of a single ADC is multiplied by the number of SHAs incircuit 300, and the required frequency of operation is similarlydivided by the number of SHAs present.

The coefficients C₀ to C_(N) input to multipliers 36 are rotated fromone multiplier 36 to the next at the sampling rate, so that each samplecaptured by an SHA 32 will be successively multiplied by eachcoefficient at successive intervals. Thus, in operation the circuit 300acts like a conventional FIR filter.

FIG. 4 is a schematic diagram of one implementation of a circuit such asthat shown in FIG. 3. In circuit 400, switches S1 to S7 close and opensequentially to distribute the input signal to the sample and holdcapacitors C1 to C7 in a round robin fashion. As this is done, thecoefficient control signals CC1 to CC7 set the appropriate coefficientvalues corresponding to the current step in the round robin sampling byadjusting variable resistors (or other impedance elements) R1 to R7, towhich the samples are passed through buffers Z1 to Z7. For simplicity,an 8 order filter is shown in FIG. 4, as in FIG. 2, although again thefilter can be of any desired order.

In operation, switch S1 will dose first, storing the current inputsignal on sample and hold capacitor C1, after which switch S1 will open.After an interval, switch S2 will dose, so that the new current sampleof the input signal is stored on capacitor C2, after which switch S2will open, while capacitor C1 now holds the input signal sample from oneinterval prior. After another interval, switch S3 will, close to storethe next sample on capacitor C3, etc.

The seventh sample will be stored on capacitor C7, with capacitor C1 nowholding the oldest sample. At this point the coefficient control signalsCO to CC7 will configure resistors R1 to R7 such that R7 is the inverseof the first desired coefficient value, R6 is the inverse of the secondcoefficient value, R5 the inverse of the third coefficient value, etc.The coefficients will thus appear in order on resistors R7, R6, R5, R4,R3, R2, and R1, respectively. Thus, as in the block diagram circuit ofFIG. 3, the newest sample is effectively multiplied by the firstcoefficient, the next sample by the second coefficient, and the oldestsample by the last coefficient.

Since C7 is the last capacitor for storing a sample, in “round-robin”operation the next, i.e., eighth, sample will be stored on capacitor C1,so that it again has the newest sample, while C7 has the sample from oneinterval prior, C6 has the sample from two intervals prior, etc. Now theresistors should be adjusted so that R1 is the inverse of the firstcoefficient value, R7 the inverse of the second coefficient value, R6the inverse of the third coefficient value, and so on, so that thecoefficients will appear in order on resistors R1, R7, R6, R5, R4, R3,and R2, respectively.

This rotation of the resistor values continues with each new sample sothat the resistor to coefficient mapping is always in an order thatresults in the newest sample being effectively multiplied by the firstcoefficient value, the next newest sample by the second coefficientvalue, etc.

While this form of rotating coefficient filter will work and avoids theneed for delay elements, the need to constantly adjust the coefficientvalues leads to some practical problems. There are digital bussesdriving the adjustable elements (here resistors), and these busses haveassociated noise; this noise tends to couple into, and thus degrade, thedesired output signal. In addition, such a system may be complex, aseach variable “resistor” (in this case) R1 to R7 is actually a set ofresistors with as many as ten or twelve switches to implement theadjustment in the resistor values described above. (There will,generally he as many switches as there are bits in the control signal,so that for a ten bit accurate adjustment this would be ten switches foreach resistive dement, etc.)

Thus, it is desirable to make a rotating coefficient filter withoutrequiring adjustable coefficient values, and with a minimum number ofswitches to reduce the noise that is injected into the digital outputsignal.

FIG. 5 shows one embodiment of a circuit that provides a rotatingcoefficient FIR filter without adjustable elements and with a reducednumber of switches. Circuit 500 of FIG. 5 implements the samefunctionality as that of circuit 400 in FIG. 4.

Switches S1 to S7 and sample and hold capacitors C1 to C7 again samplethe input signal in a round robin fashion as above, and the samples passthrough buffers Z1 to Z7 to a resistive network. Now, however, ratherthan adjustable elements, there are seven sets of resistors,representing seven sets of coefficient values, with each set havingseven resistors. Each set of seven resistors may be coupled to theoutput by one of switches Y1 to Y7, and correctly implements thecoefficients needed for one step of the determination of the desiredoutput.

Thus, resistors RA1 to RA7 are pre-selected as the inverses of theappropriate coefficients that are desired to be applied when switch S1has just closed and capacitor C1 holds the current sample. Similarly,resistors RB1 to RB7 are the inverses of the coefficients desired whenswitch S2 has just closed and capacitor C2 holds the now current sample,etc.

This means that the seven resistors RB1 to RB7 will have the sameoverall values as resistors RA1 to RA7, but the order will be rotated byone, so that RB2 will be the same value as RA1 (since the current sampleis now from C2 rather than C1), RB3 will be the same value as RA2, etc.,and RB1 will be the same value as RA7. The changing of the order of theresistor values from one set of resistors to the next effectivelycreates the rotation of coefficients accomplished by the adjustableresistors in FIG. 4.

It can thus be seen that no adjustment of the resistor values is needed,since all of the desired values are already present. All that isrequired is that the correct set of resistors be pre-selected andconnected to the output by closing the appropriate switch Y1 to Y7. Thisis done by closing one of switches Y1 to Y7 synchronously with thedosing of each of switches S1 to S7.

For example, when a first sample is taken, switches S1 and Y1 areclosed, and all other switches are open. When the second sample istaken, switches S2 and Y2 are dosed, and all other switches are open,etc., until the seventh sample is taken with switches S7 and Y7 dosedand all other switches open. For the eighth sample, again switches S1and Y1 are dosed and all other switches are open.

A rotating coefficient FIR filter may be made in this way using only around-robin sampler, with no delay elements or adjustable elements, anda minimum in number of switches that operate at the same frequency asthe switches that sample the input signal. All coefficients are alwayspresent, and only implemented by the closing of the appropriate switchconnecting the desired set of coefficients (resistors) to the output.

Such a circuit is easier to manufacture than one using adjustableelements. Each resistor represents a coefficient, and where possible maybe made as a single resistor. In some cases it may be more convenient tocreate a resistive value from a plurality of resistors; however, evenwhere this is done, no additional switch is required, thus reducing thepossibility of noise in the output signal.

It may be seen that one limitation of circuit 500 of FIG. 5 is that incircuits with large numbers of coefficients, the number of resistorsincreases as the square of the order of the filter. Thus, in a 101thorder filter having 100 coefficients, there will be 100 sets ofresistors, each having 100 resistors, for at total of 10,000 resistors,and each resistor requires space on, for example, an integrated circuit.

In some specific cases, it is not necessary to include all possible setsof coefficients, and thus the number of sets of resistors, and the totalnumber of resistors, may be reduced. For example, a rotating coefficientFIR filter may be used as an anti-aliasing filter of some kind torestrict the bandwidth of the input signal in certain digital signalprocessing applications. In such cases, and other applications of asimilar nature, the output signal rate does not always need to equal theinput signal rate.

For example, where such a filter is used as a channel selected filterfor an FM radio, the input signal may be of a frequency up to 100 MHz,thus requiring a sample rate of 200 megasamples per second (MS/s).However, only signals in the 10 MHz range are desired in the outputband. Thus, the output data rate may be as low as 40 MS/s and stilladequately represent the required 10 MHz output. This corresponds to anunder-sampling of the output by a factor of five (40 MS/s compared to200 MS/s). In such a case of under-sampling of the output, the number ofinput samplers may be selected to he a multiple of the number of outputvalues, thus allowing for the omission of sets of coefficients, and thusresistors, which will not be used.

An example of this is shown in circuit 600 of FIG. 6. In circuit 600, anunder-sampling factor of three is assumed. Thus, while there are sixsamplers (six switches S1 to S6 and capacitors C1 to C6), since theoutput will only be sampled at every third input sample, only two setsof output coefficients are required. It is assumed here that the outputwill be sampled when the first and fourth input samples are taken, i.e.,when switches S1 and S4 are closed.

A circuit such as that shown in FIG. 5 above would have six sets of sixresistors and six output switches Y1 to Y6 operating synchronously withswitches S1 to S6. However, since the output is only sampled as inputsamples are taken by switches S1 and S4, it will be apparent that fourof the sets of six resistors and the associated output switches Y2, Y3,Y5 and Y6 will never be needed. They may thus be omitted from thecircuit altogether, reducing the cost and difficulty of manufacture, aswell as the space required.

The disclosed system and method has been explained above with referenceto several embodiments. Other embodiments will be apparent to thoseskilled in the art in light of this disclosure. Certain aspects of thedescribed method and apparatus may readily be implemented usingconfigurations or steps other than those described in the embodimentsabove, or in conjunction with elements other than or in addition tothose described above.

For example, as discussed above, the elements providing the desiredimpedance values need not be resistors, but may be, for example,capacitors, inductors or FETs connected as pass devices, depletion modeMOSFETs, or other devices, with the values of the elements (such ascapacitance, inductance, etc.) selected to provide the desired impedancevalues.

It should also be appreciated that the described method and apparatuscan be implemented in numerous ways, including as a process, anapparatus, or a system. The methods described herein may be implementedby program instructions for instructing a processor to perform suchmethods, and such instructions recorded on a computer readable storagemedium such as a hard disk drive, floppy disk, optical disc such as acompact disc (CD) or digital versatile disc (MD), flash memory, etc. Themethods may also be incorporated, into hard-wired logic if desired. ItShould be noted that the order of the steps of the methods describedherein may be altered and still be within the scope of the disclosure.

These and other variations upon the embodiments are intended to becovered by the present disclosure, which is limited only by the appendedclaims.

What is claimed is:
 1. A circuit comprising; an input configured to receive an input signal; a plurality of sampling circuits arranged in parallel for sampling the input signal in response to a timing signal, the sampling circuits configured such that successive sampling circuits create samples of the input signal in time-delayed succession at pre-determined intervals; a plurality of sets of elements having impedances, each set containing the same number of elements as the plurality of sampling circuits with the impedances chosen so that a sum of outputs from the elements in each set produces a desired frequency response to the input signal that is the same for each set, with each element in a set connected to a different one of the sampling circuits in a different order from the connection of the elements in each other set, such that each set of elements will produce the desired frequency response when a different one of the sampling circuits contains a new sample of the input signal; and a plurality of switches, each switch connecting a different one of the plurality of sets of elements to an output.
 2. The circuit of claim 1 wherein the number of sets of elements having impedances is the same as the number of sampling circuits.
 3. The circuit of claim 1 wherein the number of sets of elements having impedances is less than the number of sampling circuits.
 4. The circuit of claim 1 wherein the elements having impedances are resistors.
 5. The circuit of claim 1 wherein the elements having impedances are capacitors.
 6. The circuit of claim 1 wherein the elements laving impedances are inductors.
 7. The circuit of claim 1 wherein the elements having impedances are transistors.
 8. A method of designing a finite impulse response filter having a plurality of sampling circuits arranged in parallel for sampling the input signal in response to a timing signal, the sampling circuits configured such that successive sampling circuits create samples of the input signal in time-delayed succession at pre-determined intervals, comprising: selecting a desired frequency response for the filter; selecting a plurality of sets of elements having impedances, each set containing the same number of elements as the plurality of sampling circuits with the impedances chosen so that a sum of outputs from the elements in each set produces a desired frequency response to the input signal that is the same for all of the sets, with each element in a set to be connected to a different one of the sampling circuits in a different order from the connection of the elements in each other set, such that each set of elements will produce the desired frequency response when a different one of the sampling circuits contains a new sample of the input signal; and for each set of elements, providing a switch connected to an output and to all of the elements in the set, each switch being separate from the switches connected to the other sets of elements.
 9. The method of claim 8, wherein selecting a plurality of sets of elements further comprises: determining a set of Fourier coefficients that produces the desired frequency response; and selecting a set of impedances that are the inverse of the Fourier coefficients.
 10. The method of claim 9, Wherein determining a set of Fourier coefficients further comprises mathematically calculating the set of Fourier coefficients.
 11. The method of claim 9, wherein determining a set of Fourier coefficients further comprises determining the set of Fourier coefficients by an iterative method.
 12. The method of claim 11, wherein determining the set of Fourier coefficients by an iterative method further comprises determining the Fourier coefficients by a Parks McClellan method.
 13. The method of claim 11, wherein determining the set of Fourier coefficients by an iterative method further comprises determining a set of Fourier coefficients by an iterative method performed by software that receives the desired frequency response for the filter as an input.
 14. A computer readable storage medium having embodied thereon instructions for causing a computing device to execute a method for designing a finite impulse response filter having a plurality of sampling circuits arranged in parallel for sampling the input signal in response to a timing signal, the sampling circuits configured such that successive sampling circuits create samples of the input signal in time-delayed succession at pre-determined intervals, the method comprising: selecting a desired frequency response for the filter; selecting a plurality of sets of elements having impedances, each set containing the same number of elements as the plurality of sampling circuits with the impedances chosen so that a sum of outputs from the elements in each set produces a desired frequency response to the input signal that is the same for all of the sets, with each element in a set in be connected to a different one of the sampling circuits in a different order from the connection of the elements in each other set, such that each set of elements will produce the desired frequency response when a different one of the sampling circuits contains a new sample of the input signal; and for each set of elements, providing a switch connected to an output and to all of the elements in the set that, the switch being separate from the switches connected to the other sets of elements. 